1. Field of the Invention
The present invention relates to a sensor device having a sensor element and a signal-outputting portion electrically connected to the sensor element. The sensor device is used for measuring an amount of air supplied to an internal combustion engine, for example.
2. Description of Related Art
Examples of this kind of airflow sensors for measuring an amount of air supplied to an internal combustion engine are disclosed in JP-A-7-174599 and JP-A-11-6752. The airflow sensor includes a sensing element and a signal-outputting portion electrically connected to the sensing element. The portion electrically connecting the sensing element to the signal-outputting portion is molded, while a sensing portion of the sensor element is exposed to the airflow to be measured.
In a process of manufacturing the sensor device, the sensor element and the signal-outputting portion is electrically connected, and then the sensor element and the signal-outputting portion are contained in a molding die. Then, the portion connecting the sensor element to the signal-outputting portion is molded in the molding die with a resin material. Because the connecting portion to be molded is contained in a cavity in the molding die having a certain clearance or a gap, some burs are formed on sides of the connecting portion.
This problem will be further explained, referring to FIGS. 7A, 7B and 7C showing a process of manufacturing a conventional sensor device. As shown in FIGS. 7A and 7B, a sensor element 20 having a sensing portion 21, and a circuit chip forming a signal-outputting portion 31 are mounted on a substrate 10. The sensor element 20 is electrically connected to the signal-outputting portion 31 with a bonding wire 22. A portion connecting the sensor element 20 to the signal-output portion 31 including the bonding wire 22 and the signal-outputting portion 31 are molded with a molding insulator 50.
As shown in FIG. 7C, after the sensor element 20 and the signal-outputting portion 31 are electrically connected through the bonding wire 22, the sensor element 20 is contained in a depressed portion 240 of a molding die 200 consisting of a lower die 210 and an upper die 220. Then, the connecting portion and the signal-outputting portion 31 is molded with the insulating material 50, while exposing the sensing portion 21 outside of the insulating material 50. Since a certain gap, or a clearance, G is necessary in the depressed portion 240 for accommodating the sensing element 20 therein, some of the molding insulator 50 forms burs on the sides of the sensing portion 20 in the molding process.
The gap G is necessary because the sensor element 20 is fixedly mounted on the substrate 10 and the sensor element 20 is not movable relative to the substrate 10. The burs formed on the sides of the sensor element 20 change a heat capacity of the sensor element 20, and such a change in the heat capacity affects an output of the sensor device. If the sensor device is used as an airflow sensor measuring an amount of air based on a temperature of the sensor element, the changes in the heat capacity is detrimental to the sensor output. If the sensor device is used as a diaphragm-type pressure sensor, a distortion of the diaphragm is affected by the burs formed on the sides of the sensor element. Therefore, it is necessary to eliminate or reduce such burs in the molding process.